Then Populate the values of the items in combo box . And use this collection to create the

AutoCompleteSource Property to Custom

Then set the AutoCompleteMode Property to Suggest , and set the AutoCompleteCustomSource as your collections.

For checking the items it currently checked, check the selected index property of the combox. If any thing is selected from items in the Combox, then it will be > -1 , where 0 is the first item.

MSDN Example for Auto Suggest